Prominent among the selective non-catalytic reduction (SNCR) processes are those disclosed by Lyon in U.S. Pat. No. 3,900,554 and by Arand et al. in U.S. Pat. Nos. 4,208,386 and 4,325,924. Briefly, these patents disclose that ammonia (Lyon) and urea (Arand et al.) can be injected into hot combustion gases to selectively react with NO.sub.x and reduce it to diatomic nitrogen and water.
The SNCR process described by Lyon in U.S. Pat. No. 3,900,554 reduces the concentration of nitrogen monoxide (NO) by contacting combustion gases with ammonia or certain ammonia precursors. Gas-phase reactants or aqueous solutions can be introduced into oxygen-rich waste gas for selective reaction with the nitrogen monoxide at a temperature in the range of from 870.degree. to 1100.degree. C. The limiting lower temperature of the window can be reduced by the addition of certain substances. However, there is no method or means disclosed to protect the ammonia where the temperature is too high. At temperatures above about 1050.degree. C., some of the ammonia will be oxidized to form additional NO.
Arand et al. disclose in U.S. Pat. No. 4,208,386, that urea can be added alone or in solution to oxygen-rich effluents in a temperature range from about 700.degree. to about 1100.degree. C. Again, here, as with the Lyon process, distribution is critical to selective reduction and no solution is found to the problem of high temperature operation.
In U.S. Pat. No. 4,325,924, Arand et al. describe an SNCR process utilizing urea in fuel-rich combustion effluents. They report that under these conditions, the reaction will still reduce NO.sub.x at temperatures within the range of from about 1040.degree. to about 1650.degree. C. However, fuel-rich conditions are not economical and can result in high levels of carbon monoxide.
More recently, Bowers disclosed in U.S. Pat. No. 4,992,249, that if droplet size is increased and urea concentration is decreased, good results can be achieved in oxygen-rich effluents at higher temperatures than disclosed by Arand et al. While the use of water for dilution can provide limited protection for an active agent from the intense heat, some droplets tend to be too small and evaporate too rapidly and some tend to be too large and can impact equipment. Both deviations cause reactions to occur under less than optimum conditions. And, both can result in the production and release to the atmosphere of additional pollutants--high temperatures can cause additional NO.sub.x generation, and low temperatures can generate ammonia, N.sub.2 O and carbon monoxide.
There is a present need for improved processes and compositions which are effective under oxygen-rich conditions and at highly-elevated temperatures to reduce nitrogen oxides while simultaneously maintaining low levels of secondary pollutants.